Optical printing apparatus typically focus radiation from a source, such as a laser diode, onto a radiation-sensitive recording medium typically transported in a direction substantially transverse to the direction of propagation of the radiation. To provide reproducibile amounts of focused radiation per unit area on the recording medium, it has generally been found necessary to provide an autofocus apparatus that operates continuously throughout the printing process.
Because the need for autofocus apparatus is well recognized, there have been many attempts in the art to provide such apparatus in applications calling for reading, as well as, writing on a recording medium. Specifically, there have been many attempts to provide such autofocus apparatus for receiving radiation which has been reflected from the surface of the recording medium and isolating and analyzing that radiation apart from the radiation which is incident upon the surface of the recording medium. In particular, U.S. Pat. No. 4,542,492 issued on Sept. 17, 1985 discloses an autofocusing apparatus in FIG. 1 thereof wherein radiation emitted by a semiconductor laser is substantially linearly polarized along a given direction according to the junction plane of the laser. The radiation passes through a collimating lens and a polarization beam splitter. The polarization beam splitter is fabricated, for example, as a cube from two joined or bonded prisms and the separation or splitting surface provides a polarization splitting or separating function, i.e., it transmits radiation having the given direction of polarization and reflects radiation having a polarization direction oriented at 90.degree. to the given direction. As a result, the radiation which emerges from the polarization beam splitter is substantially linearly polarized along the given direction.
Next, the radiation passes through a quarter-wave plate whose fast axis is oriented at 45.degree. relative to the polarization of the incident beam and emerges as circularly polarized radiation. The circularly polarized radiation is focused by an objective lens onto the surface of the recording medium. Radiation is reflected from the recording medium, and it is also circularly polarized, but in the opposite sense from the radiation incident thereupon. The reflected radiation passes back through the objective lens and the quarter-wave plate and, as a result of traversing the quarter-wave plate, it is converted into linearly polarized radiation whose direction of polarization is oriented at 90.degree. from the given direction. The reflected radiation then impinges on the polarization beam splitter and, because of the rotated polarization, is reflected to a photodetector means. The photodetector generates an error signal which is used to move the objective lens to maintain focus.
U.S. Pat. No. 4,654,519 issued on Mar. 31, 1987 discloses an autofocusing apparatus in FIG. 1 thereof which is similar to the one disclosed in U.S. Pat. 4,542,492 in that radiation emitted by a semiconductor laser source passes through a collimating lens, a polarization beam splitter, a quarter-wave plate after which it is focused by an objective lens onto the surface of the recording medium. The objective lens is supported, and can be moved along its optic axis to provide focusing, by means of a voice coil. Radiation reflected from the recording medium passes through the objective lens and the quarter-wave plate and, as a result of traversing the quarter-wave plate, it becomes linearly polarized with its direction of polarization being oriented at 90.degree. to the direction of the original polarization. Next, the reflected radiation impinges upon the polarization beam splitter and, because of the rotated polarization, is reflected to a photodetector having a first and a second photosensitive region. A difference signal corresponding to the difference between the electric signals from the photosensitive regions is applied to a voice coil driver to alter the position of the objective lens to maintain optimum focus. Further, the patent discloses the use of a knife edge method, well known in the prior art, as one means for providing the difference signal.
U.S. Pat. No. 4,023,185 issued on May 10, 1977 discloses an autofocus apparatus in FIG. 3 thereof wherein radiation emitted by a laser source passes through a polarization beam splitter, a quarter-wave plate and is focused thereafter by an objective lens onto the surface of the recording medium. Radiation reflected from the recording medium passes through the objective lens and the quarter-wave plate and, as a result of traversing the quarter-wave plate, it becomes linearly polarized along a direction oriented at 90.degree. to the direction of the original polarization. Next, the reflected radiation impinges on the polarization beam splitter and, because of the rotated polarization, is reflected to a photodetector means. The photodetector generates an error signal which is used to move the objective lens to maintain focus.
U.S. Pat. No. 4,358,200 issued on Nov. 9, 1982 discloses an autofocus apparatus in FIG. 3 thereof wherein a laser source emits a linearly polarized beam whose electric field vector is perpendicular to the plane of the drawing. The beam: (1) is reflected from the front surface of a plate, which front surface is a polarization sensitive splitting mirror; (2) passes through a quarter-wave plate; and (3) is focused thereafter by an objective lens onto the surface of the recording medium. Radiation reflected from the recording medium passes through the objective lens and the quarter-wave plate and, as a result of transversing the quarter-wave plate, its electric field vector is oriented parallel to the plane of the drawing, i.e., it is oriented at 90.degree. to the direction of the original electric field vector. Next, the reflected radiation impinges on the front surface of the plate and passes therethrough because the electric field vector has been rotated by 90.degree.. Finally, the radiation is reflected by the second surface of the plate to a photodetector. The photodetector generates an error signal which is used to move the objective lens to maintain focus.
Finally, U.S. Pat. No. 4,381,557 issued on Apr. 26, 1983 discloses an autofocusing apparatus in FIG. 1 thereof wherein a first laser source is used to read information stored on the recording medium and a second laser source, such as a semiconductor laser which emits coherent light at approximately 820 nm, is used to provide autofocusing. Radiation emitted by the first laser source passes through a beam telescope and a first beam splitter. Afterwards, the radiation is reflected by a dichroic reflector towards an objective lens which focuses the radiation onto the surface of the recording medium. Radiation reflected from the recording medium passes through the objective lens and is reflected by the dichroic mirror towards the first beam splitter which reflects the radiation, in turn, towards a light detector. Radiation emitted by the second laser source is focused onto a second beam splitter which transmits it towards the dichroic mirror. The radiation from the second laser source passes through the dichroic mirror and is focused by the objective lens to a fairly large spot on the surface of the recording medium. Radiation reflected from the large spot is: (1) collected by the objective lens; (2) transmitted by the dichroic mirror; and (3) reflected by the second beam splitter in a direction orthogonal to the direction of propagation of the radiation originally emitted from the second laser source to a detector.
The detector generates an error signal which is applied to a lens moving actuator for repositioning the objective lens to maintain focus for the radiation emitted by the first laser source.
As one can appreciate from the above, the above-discussed autofocus apparatus primarily rely on the fact that radiation reflected from the surface of the recording medium can be separated from radiation incident upon the recording medium by introducing some asymmetry in the optical path between the laser source and the surface of the recording medium. However, the disclosed autofocus apparatus further rely on the fact that there is only one source of reflected radiation, i.e., one surface of the recording medium. As a result, the disclosed apparatus are not suitable for use with a recording medium comprised, for example, of relatively thin layers of two different materials. This is because such a recording medium causes reflection of incident radiation from the front surface of the first material and from the back surface of the first material at the interface between the first and the second materials and the disclosed apparatus cannot separate and distinguish the radiaton reflected from these two surfaces. Thus, the disclosed autofocus apparatus cannot be used to focus on either the first or the second of these two surfaces.
In particular, consider the following specific example of a recording medium comprised of relatively thin layers of two different materials, i.e., a recording medium comprised of a thermally sensitive material such as carbon which is coated onto a transport material such as a 178 um thick Mylar sheet. In operation, radiation from, for example, a laser diode operating at a wavelength substantially at 820 nm, passes through the Mylar sheet and is focused on the interface between the Mylar and the carbon backing. A portion of the radiation is absorbed in the carbon backing and the heat generated thereby causes the carbon to bond to the Mylar. Because of the composition of the recording medium, radiation is reflected from the surface of the carbon backing as well as from the front surface of the Mylar sheet.
In practice, the above-described problem is exacerbated by the fact that the reflectivity of the Mylar sheet is several times larger than the reflectivity of the carbon. For example, in practice, the optical signal reflected from the Mylar sheet is approximately five times larger than that reflected from the Mylar-carbon interface located at the back, second surface of the 178 micrometer thick Mylar sheet. This causes a substantial problem because laser radiation must be focused at the surface of a material, namely, carbon, which reflects a signal which is approximately one-fifth as large as the unwanted signal reflected by the other material, namely, Mylar. Further, because the Mylar sheet sheet is so thin, the desired focus error signal due to the second surface, i.e., the Mylar carbon interface, is normally superimposed upon the undesired focus error signal due to the first surface, i.e., the air Mylar interface, and, as a result, the desired focus error signal is swamped by the undesired signal. Thus, acquisition and tracking of the second surface is not practicable with the above-described autofocusing apparatus disclosed in the prior art. In addition to the above, another fact that must also be considered is that the transport medium used in this particular application, namely, Mylar, is birefringent.
In light of the above, there is a need in the art for an autofocus apparatus for use with a recording medium comprised of a radiation sensitive medium affixed to a birefringent transport medium wherein radiation from a light source is focused through the birefringent transport medium and onto the interface between the birefringent transport medium and the radiation sensitive medium.